As Rick Perry and Mitt Romney jockey over their ability to defeat President Barack Obama, there are deepening fault lines between the two on Social Security, immigration, jobs and more that could shape the contest.

Their stylistic differences are as stark as their disagreements on substance. Romney, the former Massachusetts governor, also is a former venture capitalist who is at his best when he's talking about how to help businesses help the economy grow. Perry, the Texas governor, is a fiery, red-meat conservative who has already shown he loves to go on the attack — and isn't afraid to go after his chief GOP rival.

Those contrasts have driven Romney's campaign to fundamentally change a strategy that was previously aimed squarely at Obama. Until Perry jumped into the race and almost immediately displaced Romney as the front-runner, the former Massachusetts governor focused his public appearances and messaging on the president.

Now, instead of running a general election campaign in primary season, Romney will spend the early months trying to convince Republicans that Perry can't beat Obama in November.

It will start with Social Security, an issue Romney's campaign has decided is Perry's biggest liability. Aides privately say they plan to make it a singular focus in the coming weeks.

"You say that by any measure, Social Security is a failure. You can't say that to tens of millions of Americans who live on Social Security and those who have lived on it," Romney said in Wednesday night's debate, after he and Perry had already traded jabs over their jobs records earlier in the debate.

The Romney campaign has followed that with a steady stream of press releases, background material and on-the-record quotes assailing Perry as a career politician who is unelectable.

"If (Perry) were to win the nomination, the most interesting thing that it would prove is that God is a Democrat," said Stuart Stevens, a top Romney strategist.

Romney has also started to take on Perry's immigration record. Advisers say that could be the next front in the fight, largely because it could hurt Perry with the conservative base he relies on. As governor of a border state with problems related to illegal immigration, Perry has said a physical border fence isn't necessary. Texas universities also allow the children of illegal immigrants to pay in-state tuition rates.

Romney used a recent immigration address to emphasize his support for a fence on the southern border — and to highlight his veto of an in-state tuition bill in Massachusetts

But putting so much focus on such issues carries risks: They're a distraction from the central, disciplined jobs-and-economy message that Romney has been pushing steadily for months. The economy is the issue most likely to drive voters in 2012.

Perry has already made clear that he will run primarily on his jobs record — Texas gained more than 1 million jobs during his tenure as governor. And he has taken almost every opportunity to go after Romney on the issue.

"Michael Dukakis created jobs three times faster than you did, Mitt," Perry jabbed on the debate stage. And after Romney unveiled a 160-page economic recovery plan earlier this week, Perry's campaign immediately put out a statement slamming it.

Speaking of his GOP opponents, Perry told Republicans in California on Thursday, "We got our differences and we'll talk about them and what have you and hopefully in a very respectful way." Most important, he said, "is we need to have a nominee that doesn't blur the lines between themselves and the current resident of the White House."

Perry's message is aimed squarely at an angry GOP base that's clearly hungry for a candidate who isn't afraid of a fight. He's willing to deliver strong rhetoric on issues like the death penalty, a subject that drew applause at the debate.

Romney, by contrast, is positioning himself as the technocratic, business-friendly candidate in the race. He is clearly most excited when he's talking about what he calls his "business plan" for the country. The last time he was confronted with questions about Social Security, he ended up talking about corporations.

"Corporations are people, my friend," he exclaimed in August at the Iowa State Fair, staring down a heckler in the crowd who wanted to know how Romney was going to protect citizens on Social Security instead of looking out for companies.

These contrasts are set to shape a primary race that's still in its early stages, with more than four months before voters go to the polls in Iowa, New Hampshire and South Carolina. And the dynamic is likely to push out the myriad of other candidates who have spent brief periods in the spotlight in recent months.

Chief among them is Minnesota Rep. Michele Bachmann, whose campaign has wobbled since she won a key test vote in Iowa in August. She's lost top members of her campaign team — and the media attention that had raised her political profile. Perry clearly eclipsed her onstage on Wednesday, and his expected attractiveness in Iowa, where she must win, threatens to doom her candidacy.

Instead, Perry's entry could turn Iowa, the leadoff caucus state, into a two-person showdown. Romney has been relatively quiet there so far, but aides say he could up the ante in the state.

"Perry has to win here," said Doug Gross, who is undecided but was a top Romney supporter in 2008. "What Romney needs to do right now in Iowa is make sure that Perry can't get a big win early."

And Perry could find a footing in New Hampshire, as well, where Romney had been running largely unopposed but where many conservatives are still looking for an alternative candidate.

"People were really hungry for an alternative, and no one has filled that niche," said Rich Killion, a New Hampshire-based strategist who was working for former Minnesota Gov. Tim Pawlenty until he dropped out of the race. "With Perry getting into the race, he's completely filled that vacuum. And it is a two-man race."

Still, there are months before the first votes are cast, and there are still two debates coming up in September alone. That leaves plenty of room for mistakes, and for Romney to start spending millions of dollars in an attempt to define Perry for voters who are just getting to know him.

"It's important to remember that the first contest is five months away and anything can happen," said Mike Dennehy, a longtime GOP strategist. "We shouldn't' write anyone off until the first votes are cast."